In recent years, with the rapid spread of information-related devices, communication devices and so on, great emphasis has been placed on the development of batteries for use as the power source of such devices. Also, in the automobile industry, the development of high-output and high-power batteries for electric vehicles and hybrid vehicles, has been promoted. Of various kinds of batteries, a lithium battery has attracted attention, due to its high energy density and high power output.
In lithium batteries, generally, a lithium metal complex oxide which has a layered structure, such as lithium nickelate or lithium cobaltate, is used as the cathode active material, and a carbonaceous material which is able to occlude/release lithium ions, a lithium metal, a lithium alloy or the like is used as the anode active material. As the electrolyte disposed between the cathode and the anode, a liquid electrolyte in which lithium salt is dissolved, a solid electrolyte which contains lithium, etc., is used.
As described above, while lithium batteries have excellent energy density and power output, rising lithium prices due to a growing demand for lithium batteries, limited lithium reserves, etc., are a bottleneck in mass production and upsizing.
Accordingly, studies on sodium batteries have been promoted, in which sodium, which is an abundant resource and low-cost, is used in place of lithium (e.g., Patent Literatures 1 and 2).
For example, a cathode active material for sodium batteries is disclosed in Patent Literature 1, which is represented by a general formula NaxMy(AO4)zP2O7 in which M is any of Ni, Mn and Co, and a part thereof can be substituted with at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Also in Patent Literature 1, the synthesis and evaluation of a cathode active material are described, which is represented by the above general formula in which M is Co and a part thereof is substituted with at least one of Mn and Ni.